1. Technical Field
The present disclosure relates to semiconductor fabrication, and more specifically, to forming uniform depth fin trenches and fins.
2. Related Art
In semiconductor devices, semiconductor fins are used for a wide variety of devices such as aspect ratio trapping structures and fin field effect transistors (finFETs). One challenge in forming the fins is ensuring a uniform height of the fins and/or uniform aspect ratios of the fins. As shown in FIG. 1, fin formation typically includes photo-lithographically forming a number of trenches 10 in a substrate 12. In photolithography, a radiation sensitive “resist” coating is formed over one or more layers which are to be treated, in some manner, such as to be selectively doped and/or to have a pattern transferred thereto. The resist, which is sometimes referred to as a photoresist, is itself first patterned by exposing it to radiation, where the radiation (selectively) passes through an intervening mask or template containing the pattern. As a result, the exposed or unexposed areas of the resist coating become more or less soluble, depending on the type of photoresist used. A developer is then used to remove the more soluble areas of the resist leaving a patterned resist. The patterned resist can then serve as a mask for the underlying layers which can then be selectively treated, such as to undergo etching to form trenches. As also shown in FIG. 1, trenches 10 are filled with sacrificial material pillars 14 by depositing the material and planarizing.
As shown in FIG. 2, a reactive ion etch (RIE) is then performed to create a starting surface 16 on substrate 12 between adjacent pillars 14 upon which fins will be epitaxially grown. RIE is a variation of plasma etching in which, during etching, the semiconductor wafer is placed on an RF powered electrode. The plasma is generated under low pressure (e.g., vacuum pressure) by an electromagnetic field. It uses chemically reactive plasma to remove material deposited on wafers such as substrate 12 between pillars 14. High-energy ions from the plasma attack the wafer surface and react with it. The wafer takes on potential which accelerates etching species extracted from plasma toward the etched surface.
One challenge with RIE is that attaining uniform etching is very difficult because the process provides no endpoint detection and typically relies exclusively on controlling the duration of the process to attain depth uniformity. However, as shown in FIG. 2, even identical materials typically etch at different rates—the RIE does not create surfaces 16 between pillars 14 having uniform height. As a result, any fin grown from surfaces 16 will inherently have different heights and different aspect ratios. This problem is oftentimes magnified because RIE chambers exhibit different etch rates over time and between periodic maintenance.